A color graphics terminal having a cathode-ray-tube (CRT) display monitor generally includes a frame buffer in which information representing the distribution of color over the screen of the CRT is stored. The addressable memory locations in the frame buffer are associated on a one-to-one basis with the pixels of the display, and each addressable location is capable of storing a digital word comprising four binary digits or bits, representing one of 16 possible colors for the associated pixel. The four-bit words are read from the frame buffer in timed relationship to the scanning of the terminal's display screen, to provide a four-bit parallel signal. The four-bit parallel signal is converted into three two-bit parallel signals. Each set of three two-bit words represents the contributions of three primary color components (red, green and blue) to the color of the associated pixel. Two one-bit signals, representing blanking information and sync information respectively, are also generated. The three two-bit color signals and the one-bit blanking signal are logically combined in such manner that if blanking is asserted the color signals are each driven to 00 whereas if blanking is not asserted the color signals are unchanged. The three two-bit color signals, modified by the blanking signal, are applied to a DAC system in which they are compared with reference voltages, and three analog signals are generated. The one-bit sync signal is used to add sync information to at least one of the analog signals during the blanking interval, and the resulting three analog signals are applied to respective output amplifiers. Each amplifier either has an output impedance of 75 ohms or is connected to an impedance matching network having an output impedance of 75 ohms, in order to facilitate matching to the monitor input terminals. The analog output signals of the three amplifiers are used to drive the three electron guns of the CRT.
The conventional color video DAC system is subject to the disadvantage that it has a high parts count and a large number of different parts. Accordingly, manufacture and calibration of the conventional DAC system is expensive.